dbx Functionality Available for OpenMP Code

In addition to the usual functionality for debugging multithreaded programs, dbx allows you to do the following in an OpenMP program:

• Single step into a parallel region. Because a parallel region is outlined and called from the OpenMP runtime library, a single step of execution actually involves several layers of runtime library calls that are executed by slave threads created for this purpose. When you single step into the parallel region, the first thread that reaches the breakpoint causes the program to stop. This thread might be a slave thread rather than the master thread that initiated the stepping.

  For example, refer to the Fortran code inHow Compilers Transform OpenMP Code, and assume that master thread t@1 is at line 10. You single step into line 12, and slave threads t@2, t@3, and t@4 are created to execute the runtime library calls. Thread t@3 reaches the breakpoint first and causes the program execution to stop. So the single step that was initiated by thread t@1ends on thread t@3.This behavior is different from normal stepping in which you are usually on the same thread after the single step as before.

• Print shared, private, and threadprivate variables. dbx can print all shared, private, and threadprivate variables. If you try to print a threadprivate variable outside of a parallel region, the master thread’s copy is printed. The whatis command does not tell you whether a variable is shared, private, or threadprivate.